1. Field of the Invention
The present invention relates to a method for manufacturing a Silicon-On-Insulator (SOI) substrate having an oxide film which is partially buried inside a silicon substrate with a separation by implanted oxygen (SIMOX) method.
2. Background of the Invention
Conventionally, SOI substrates having a buried oxide film inside a silicon substrate have been expected to be used as a substrate for a high speed and low power device. Among these, an SOI substrates having a buried oxide film , not entirely, but partially inside the silicon substrate (hereinafter, referred to as a “partial SOI substrate.”) have become increasingly valued in an LSI system on which an analog portion, a logic portion, and a memory portion are combined. This is because, for example, only the logic portion is formed in the SOI region of the buried oxide film, and the memory portion can be formed in a bulk Si portion without the buried oxide film, or the like.
A method for manufacturing this kind of partial SOI substrate, i.e., a method for manufacturing SIMOX substrate, is disclosed in Japanese Unexamined Patent Publication No. H5-82525 (see claim 2). As shown in FIG. 2, a mask oxide film 4 is first formed on a surface of a substrate 2 (The substrate 2 is cut in a plane perpendicular to an axis of a silicon single crystal rod.) (FIG. 2(a)), and a resist layer 6, patterned by photo lithography, is formed on a surface of this mask oxide film 4 (FIGS. 2(b) and 2(c)). Subsequently, the mask oxide film 4 is patterned by anisotropic etching (FIGS. 2(d) and 2(e)), and after removing the resist layer 6, (FIG. 2(f)), the substrate 2 is cleaned. Next, after implanting oxygen ions 7 into the surface of the substrate 2 (FIG. 2(g)), the substrate 2 is immersed in an etching solution of a mixture of a fluoric acid ammonium solution and a fluoric acid, and then the mask oxide film 4 is removed (FIG. 2(h)). Further, after subjecting the thus treated substrate to an annealing treatment while holding it for a predetermined time at 1300° C. or more in an atmosphere of a mixed gas of argon and oxygen, or a mixed gas of nitrogen and oxygen to thereby form a buried oxide film 3 (FIG. 2(i)), the substrate 2 is immersed in an etching solution of a mixture of a fluoric acid ammonium solution and a fluoric acid, and surface oxide layer 8 is removed (FIG. 20)).
However, according to the aforementioned conventional method for manufacturing the SIMOX substrate, as shown in FIGS. 2(i) and 2(j), since the volume of the oxygen ion region 9 serving as the buried oxide film 3 expands during the annealing treatment after implanting the oxygen ions 7, the substrate surface 2a serving as an SOI region expands more than the substrate surface 2b serving as a bulk region, and thus the problem arises that a step forms on a surface of the substrate 2 after removing the surface oxide layer 8.
Moreover, according to the aforementioned conventional method as shown in FIG. 2(g), a recess 2c is locally formed on the surface of the substrate serving as the SOI region due to sputtering of the oxygen ions 7 during the oxygen ion implantation. Consequently, concerns have arisen that the thickness of the buried oxide film 3 after the annealing treatment has changed, or the buried oxide film 3 has been exposed on the surface of the substrate 2 after the annealing treatment as shown in FIG. 2(j).